The anticorrosive property requirements imposed on products and components are becoming ever more stringent. For example, ten years ago, an acceptable corrosion resistance level under salt spray tests for the most widely accepted and commercially used anticorrosive zinc coating was the 75-hour corrosion resistance level or, as a maximum, the 150-hour corrosion resistance level. Today, the requirement may be a 400-hour corrosion resistance level or greater. (The salt spray test data results cited herein were obtained using standard ASTM B 117).
Anticorrosive coating manufacturers have attempted to meet the more stringent coating corrosion resistance requirements by the addition of numerous components aimed at increasing the corrosion resistance of their coating compositions.
In the electroplating process, coatings composed of Zn—Fe, Zn—Ni, Zn—Co, Zn—Cd alloys were developed and produced. By using chromate passivation, it became possible to maintain coating corrosion resistance for 400 to 500 hours, before red corrosion traces appeared in the salt spray test. Red corrosion traces indicate the beginning of iron-based substrate corrosion.
However, the added complexity and sensitivity of the coating precipitation process and the resulting need for more precise production control led to a substantial increase in coating production costs. In addition, the addition of environmentally dangerous elements, such as Ni, Co, and Cd, in the coatings led to higher waste treatment expenditures.
Another commonly accepted method for applying a corrosion resistant zinc-based coating is the hot-dip (or galvanizing) process; in which treated parts are immersed in a molten zinc bath. This method is widely used for applying coatings onto large-sized products, and also for a continuous process of applying protective coatings on metal sheet and wire.
However, the hot-dip zinc coating, thus applied, exhibits a relatively low corrosion resistance, (a corrosion resistance of approximately 150 hours was maintained before the appearance of red corrosion in the salt spray test) and high white corrosion susceptibility. White corrosion refers to the corrosion products of zinc.
In the 1970s, hot-dip processes were modified to use a bath containing aluminum, as well as zinc. Coatings produced using these hot-dip processes have various commercial names, however, their chemical composition is essentially the same. The coating, known in Europe and the USA as Galfan, contains 4.7–5.2% by weight of aluminum. The Galvalum coating, commercially produced in the USA, contains approximately 55% by weight of aluminum.
Galfan coating is characterized by a 3 to 5-fold lower corrosive mass-loss rate than that of conventional hot-dip technology coatings, and also, by an increased period of time elapsed before the appearance of red corrosion of the base metal.
Although the introduction of relatively small amounts of aluminum (usually 4–7% by weight) is known to lead to an increased corrosion resistance of zinc coating, the electrochemical mechanism that leads to this effect is not apparent.
The higher resistance of Zn—Fe, Zn—Ni, Zn—Cd alloys in comparison with pure zinc can be attributed to their higher electrode potential, as the electrochemical potential of the second component of the alloy is higher than that of zinc, as shown in the table below:
TABLE 1Electrochemical Potential of Alloy ComponentsElementElectrochemical potential, VZn−0.763Fe−0.037Ni−0.250Cd−0.403Al−1.663Mg−2.37
Conversely, in zinc alloys with aluminum and magnesium having electrochemical potentials equal to −1.663 V and −2.37 V, respectively, the situation is reverse. From the electrochemical point of view, zinc alloys with aluminum and/or magnesium should be less corrosion resistant.
According to J. Tanaka et al., “Effect of Mg and Si on the Microstructure and Corrosion Behavior on Zn—Al Hot Dip Coatings on Low Carbon Steel” (ISIJ international, Vol. 42 (2002), N1, pages 80–85), a Zn—Al coating with Mg and Si admixtures has a complicated phase composition caused by phase transformation processes at the transition from bath temperature to room temperature. In addition to a Zn—Fe intermetallide layer, adjacent to the base metal, phases of almost pure zinc, almost pure aluminum, and eutectic Zn—Al compositions, with approximately 5–6% by weight of Al, are also observed. According to these authors, almost pure aluminum does not corrode due to its self-passivation capability.
The increased corrosion stability of Galfan type coatings seems to be due to the fact that the eutectic aluminum, when subjected to corrosion, forms insoluble compounds, which fill coating defects, such as, pores and cracks. In essence, the activity of these insoluble compounds provides an effect of coating self-passivation similar to that observed in pure aluminum and its alloys.
The increase in the corrosion resistance of zinc-aluminum coatings, produced by hot-dip technology, has been achieved by making the chemical composition of the coating more complex. Adding magnesium and silicon to the coating composition made it possible to bring the corrosion resistance of such coatings up to the level of today's industrial anticorrosive coating requirements.
The modified hot-dip technology process, however, requires an extremely delicate multistage pretreatment of surfaces to be coated, fluxing, and in many cases, chromate passivation of finished coating as well. There are also problems in maintaining a constant melt composition in the bath, and, as a rule, the coating itself is applied in several steps.
To attain a high coating adhesion to the substrate, it is necessary to introduce various kinds of admixtures into the bath, including rare-earth elements. This increases 2 to 2.5-fold the cost of polymetallic coating in comparison with a conventional hot-dip coating.
Coatings applied, using this technology, on parts of complex shape fail to provide a uniform thickness.
This technology is inapplicable for parts having blind holes and internal thread.
It is difficult to automate the process of coating small-sized parts. Applying coatings onto small flat parts, for example, washers, presents a problem as well.
The hardness level of zinc-based coatings, applied by both the electroplating and hot-dip processes, is not high. It is 50–65 HV for zinc, zinc-aluminum and other zinc alloy coatings. A low hardness level leads to a rapid wear of coating in friction and erosion areas and, consequently, to the deterioration of corrosion stability.
Another common drawback of zinc-based coatings, applied by both the electroplating and hot-dip processes, is their low adhesion to secondary coatings. The secondary coatings are applied to increase the corrosion resistance, to attain the required appearance of the product, and to obtain the specified technical characteristics, for instance, to increase or decrease the friction coefficient. White corrosion of zinc, developed in the vicinity of pores and defects of the second coating, leads to the mechanical destruction of the second coating. As the corrosion process advances, its products displace the second coating, which exhibits blistering.
The above-mentioned process drawbacks are eliminated using diffusive metal coatings. According to this technology, a substrate to be coated is placed into a powdered metal medium and heated up to a temperature, at which diffusion of atoms occurs between the substrate metal and the powder on the substrate surface. A particular version of this method widely used in industry is Sherardizing.
According to the established understanding, Sherardizing is a process where parts are heated for several hours in a closed, usually rotating, container together with zinc powder at temperatures of 370–450° C. As a result of this process, two intermetallide Zn—Fe phases are formed on the substrate surface. The first phase is usually only several microns thick. It is adjacent to the base metal and contains approximately 20% by weight of iron. The second phase, which forms the main part of the coating thickness, contains less iron, usually up to 12% by weight of iron.
The process temperature approaches the zinc melting temperature (˜419° C.) and sometimes exceeds it. To prevent the powder from fusing and/or sticking to the substrates, the zinc powder is diluted with inert filler, such as, sand, aluminum oxide, and so forth. Alternately, according to Soviet Union patent SU 1534091 to Galin et al, the surface of zinc powder particles may be treated by a special hydrothermal method, which creates a layer that prevents the zinc powder particles from fusing.
As a result of the Sherardizing process, a relatively rough and porous coating is formed, which adheres uniformly to the profile of the substrate. The coating also serves as an excellent substrate, upon which to apply a second coating. The Zn—Fe intermetallide hardness level is high, 280–400 HV, and therefore prevents rapid wear of coating in friction and erosion areas and, consequently, maintains corrosion stability.
The corrosion resistance of the intermetallic Zn—Fe, containing approximately 12% by weight of Fe, would be expected to exceed that of electrochemically applied Zn—Fe alloy coatings. The electrochemically applied Zn—Fe alloy coatings exhibit a salt spray test corrosion resistance of up to 120 hours (and as high as 300 hours, after chromate passivation) at a thickness of 5 microns.
The Sherardizing process applied coating thickness may reach tens of microns and even exceed 100 microns; consequently, it would be natural to expect unique anti-corrosive properties of this coating. However, this is not the case. In a moist atmosphere, a layer of white corrosion covers a newly applied coating, obtained by using the Sherardizing process, in just a few hours. This white corrosion is a result of contraction cracks reaching the base metal. These contraction cracks are formed in the coating during the cooling-down of coated products.
Galvanic couples are rapidly formed in a moist, natural environment, promoting intense corrosion of the coating. Consequently, at present, all substrates coated using the Sherardizing method are either phosphatized and/or covered with a second protecting layer, for example, Dacromet paints. Phosphatization of substrates leads to an increase in salt spray test corrosion resistance levels. Corrosion resistance levels of up to 150–250 hours until the appearance of white corrosion, and up to 400 hours and more when organic coatings are applied, have been obtained.
The corrosion resistance of such a coating cannot be appreciably improved, not even by coloring with metal oxides and compounds introduced into the coating by the method suggested by U.S. Pat. No. 6,171,359 B1 to Levinski et al. The maximum salt spray test corrosion resistance of the coating, obtained using the Levinski patent, was only 192 hours until the appearance of yellow corrosion. Yellow corrosion refers to corrosion observed in a coating containing Fe. These relatively low corrosion resistance levels are due to the fact that these metal oxides coloring techniques slow down, but fail to entirely eliminate white corrosion development in the main coating, as described previously.
The prior art does not describe improvements to the Sherardizing coating corrosion resistance by means of alloying the coating with different chemical elements. Probably, this is due to the fact that the structure of the Zn—Fe intermetallic, in contrast to alloys, has a limited capability of alloying with other chemical elements.
Therefore, it would be desirable to provide a diffusion coating that possesses the advantages of the Sherardizing process, but attains a high corrosion resistance, as well. In addition, it would be desirable to attain specific unique properties in this coating by including other metals, such as, tin, silicon, and magnesium, in the coating composition.